Activity in foreground task should be able to launch activity

Assume a visible task contains 2 activities:
 Activity B, process X (top-visible)
 Activity A, process Y
Though process Y doesn't have visible activity, it shouldn't be
blocked to launch activity because it is in a visible task.

This restores the accidental behavior change in commit 9b7603b5.

Also simplify rankTaskLayers to reduce unnecessary anonymous class
and duplicated states (changed/scheduled).

Bug: 182694325
Test: atest RootWindowContainerTests#testTaskLayerRank
            BackgroundActivityLaunchTest# \
            testActivityNotBlockedFromBgActivityInFgTask
Change-Id: Ia8739967805aa8a43f99ab18840b02ac50380912